gocui: swallow the version-only diff-metadata handshake OSC

A metadata-aware pager emits a version-only OSC 1717 record (no fields) as its
first output, to announce it speaks the diff-line-metadata protocol so lazygit
can probe for it. It isn't per-line metadata, so on a real render it must be
swallowed whole rather than lingering in the accumulator and attaching to the
following diff-header line (or, if newline-terminated, producing a phantom blank
line). Drop any OSC 1717 payload with no fields at its terminator; per-line
payloads always have fields, so they're kept.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pkg/gocui/escape.go

@@ -485,8 +485,10 @@ func (ei *escapeInterpreter) parseOne(ch []byte) (isEscape bool, err error) {
 	case stateOSCMetadata:
 		switch {
 		case characterEquals(ch, 0x07):
+			ei.dropMetadataIfHandshake()
 			ei.state = stateNone
 		case characterEquals(ch, 0x1b):
+			ei.dropMetadataIfHandshake()
 			ei.state = stateOSCEndEscape
 		default:
 			ei.metadata.Write(ch)
@@ -507,6 +509,18 @@ func (ei *escapeInterpreter) parseOne(ch []byte) (isEscape bool, err error) {
 	return false, nil
 }
 
+// dropMetadataIfHandshake discards a just-completed OSC 1717 payload that carries no
+// fields (no ';'). A metadata-aware pager emits such a version-only record once, as
+// its first output, to announce it speaks the protocol (so we can probe it; see the
+// raw-diff fallback in the lazygit staging helper). It isn't per-line metadata, so it
+// must not linger in the accumulator and attach to the following line. Per-line
+// payloads always have fields, so they're kept.
+func (ei *escapeInterpreter) dropMetadataIfHandshake() {
+	if !strings.Contains(ei.metadata.String(), ";") {
+		ei.metadata.Reset()
+	}
+}
+
 func (ei *escapeInterpreter) outputCSI() error {
 	n := len(ei.csiParam)
 	for i := 0; i < n; {

pkg/gocui/view_test.go

@@ -217,6 +217,42 @@ func TestDiffLineMetadataPayloads(t *testing.T) {
 	}, v.DiffLineMetadataPayloads())
 }
 
+func TestDiffLineMetadataHandshakeSwallowed(t *testing.T) {
+	v := NewView("name", 0, 0, 80, 10, OutputNormal)
+
+	// A metadata-aware pager emits a version-only handshake (an OSC 1717 with no
+	// fields) as its first output, immediately before the diff, to announce it speaks
+	// the protocol. It must be swallowed whole: no visible bytes, no phantom line, and
+	// crucially it must not attach as metadata to the diff header that follows it.
+	osc := func(payload string) string { return "\x1b]1717;" + payload + "\x1b\\" }
+	v.writeString(osc("1") + strings.Join([]string{
+		"diff --git a/foo.txt b/foo.txt",
+		osc("1;a;1;;foo.txt") + "added",
+	}, "\n"))
+
+	// The handshake produced no phantom line and no visible bytes.
+	assert.Equal(t, []string{
+		"diff --git a/foo.txt b/foo.txt",
+		"added",
+	}, v.BufferLines())
+
+	// The handshake didn't bleed onto the header line, and the real per-line metadata
+	// after it still applies.
+	type result struct {
+		payload string
+		ok      bool
+	}
+	got := make([]result, len(v.buf.lines))
+	for y := range v.buf.lines {
+		payload, ok := v.DiffLineMetadataInLine(y)
+		got[y] = result{payload, ok}
+	}
+	assert.Equal(t, []result{
+		{"", false},
+		{"1;a;1;;foo.txt", true},
+	}, got)
+}
+
 // When a re-render produces fewer view lines than the previous one,
 // refreshViewLinesIfNeeded must truncate viewLines to the new content. If it
 // didn't (it used to overwrite in place and keep the tail), a reader could map a